In downhole drilling and completion (for example gas and oilfield exploration and production, carbon dioxide sequestration, etc.) elastomers are used in applications as diverse as packer elements, blow out preventer elements, O-rings, gaskets, and the like. The elastomers are often exposed to high temperatures and harsh chemical and mechanical subterranean environments that can degrade elastomer performance over time, reducing their reliability.
An elastomer having good chemical resistance maintains its mechanical properties, for example elasticity, extrusion resistance, and integrated structural strength, when it is contacted with various chemicals. In downhole drilling and completion applications, these chemicals include various corrosive water- and oil-based downhole fluids. Thus, in the oil and gas industry, it is more important to for an elastomer to maintain its mechanical properties under “wet” rather than under “dry” conditions at given temperature and service time.
Even with the most recent technologies, there nonetheless remains a need for elastomers, or any other polymeric materials, that function well and maintain their mechanical properties at high temperatures under wet conditions. High temperature polymers that are chemically resistant under dry conditions alone are readily available. Such polymers include certain thermoplastic polyimides (TPI) and polybenzimidazoles (PBI). Chemically resistant polymers useful under wet conditions at low temperature are also readily available. Examples of these polymers include certain polyethylenes and polypropylenes. Under conditions of high temperature and corrosive fluids, fluoropolymers are often used, as they are generally considered to have the best thermal stability and chemical resistance. Examples of fluoropolymers include polytetrafluoroethylene, and certain other fluoroelastomers and perfluoroelastomers. Certain grades of fluoropolymers are claimed to have a maximum continuous service temperature of 327° C. However, even the best perfluoroelastomers can become soft at high temperature over time, losing their capability to seal gaps under high pressure. Also, fluoroelastomers or perfluoroelastomers tend to develop cracks when contacted with various downhole fluids at high temperature.
Other types of polymers such as polyetheretherketone (PEEK) or polyphenylene sulfide (PPS) have been widely used in downhole environment as the backup rings. These polymers are rigid semi-crystalline thermoplastics and can withstand high heat and exposure to caustic chemicals. However, these polymers lack elasticity and they are not desirable to be used as sealing materials. Furthermore, it is found that these polymers tend to become brittle and break apart when contacted with various corrosive downhole fluids at high temperature.
Other polymeric materials, for example linear amorphous thermoplastics such as polysulfone are known and widely used as adhesives, composites, or moldings for automobiles, household appliances, and other applications. However, linear amorphous thermoplastics tend to creep under load, especially at elevated temperatures. Furthermore, these polymeric materials are sensitive to various solvents, which significantly limits their use in downhole drilling and completion. Attempts to modify the properties if polysulfones have included crosslinking. For example, U.S. Pat. No. 4,431,761 discloses a method to chemically replace the end groups of hydroxyl-terminated polyethersulfone to provide ethynyl-terminated polyethersulfones that can then be thermally crosslinked. U.S. Pat. No. 4,414,269 discloses a method to functionalize polysulfones with the condensation products of amino-phenols and acid anhydrides, which are thermally crosslinkable. However, these methods require additional chemical reaction steps involving expensive chemicals and solvents. Furthermore, these methods are limited to polysulfones having functional end groups such as hydroxyl-terminated polyethersulfone. Polyethersulfone tends to degrade and becomes brittle in various corrosive downhole fluids at elevated temperature. Other polysulfones such as polyphenylene sulfone (PPSU) have a better chemical resistance than polyethersulfone. PPSU can be crosslinkable via a thermal oxidation process by adding a small amount of an oxidant such as a peroxide. This crosslinked PPSU exhibits good high temperature (250° C. or above) rubbery behavior under dry conditions, but when contacted with aggressive corrosive downhole fluid, it tends to become brittle and break apart.
Despite extensive research directed to replacing elastomers or increasing their resistance to degradation under downhole conditions, there remains a need in the oil and gas drilling and completion industry for elastomers having improved chemical resistance, particularly at high temperatures. It would be a further advantage if the improved chemical resistance could be obtained without significantly adversely affecting other desirable properties of the elastomers for downhole applications, for example mechanical properties such as elasticity, extrusion resistance, and integrated structural strength. There remains a particular need for elastomers useful in devices such as packers, blow out preventer elements, O-rings, gaskets, and the like that retain good mechanical properties at high temperature when in contact with corrosive downhole fluids over continuous service times.